A resistor mat having a plurality of pressure-sensitive resistor elements is described in German Patent No. 42 37 072. Such a resistor mat is integrated in the vehicle seat, for example in the front passenger seat, in order to automatically detect seat occupancy. Whether or not the airbag belonging to the front passenger seat is to be deployed in the event of a crash or what inflating intensity is desirable basically depends on the occupancy of the front passenger seat. If no occupancy or a child's seat is sensed on the vehicle seat by the resistor mat, the deployment of the airbag is to be completely suppressed. The same is true if the front passenger seat is occupied not by a person but by an object (e.g., a piece of baggage). The intensity of inflation depends on the size of the person occupying the vehicle seat, which is expressed by his or her weight, which can be measured using the resistor mat. The resistor mat can also provide information on the seating position of the person, which should affect the inflation intensity of the airbag. The more accurately the resistances of the pressure-sensitive resistors arranged in the resistor mat in a matrix form can be measured, the more accurate the information on the type of occupancy or seating position of a person on the vehicle seat.
An object of the present invention is to provide a circuit arrangement which performs an accurate resistance measurement of the individual resistors arranged in a matrix form in a pressure-sensitive resistor mat with the lowest possible circuit complexity.
In an article by T. D'Alessio, “Measurement Errors in the Scanning of Piezoresistive Sensor Arrays,” Sensors and Actuators A, CH, Elsevier, Lausanne, Vol. 72, No. 1, Jan. 8, 1999, pp. 71-76, different circuits are shown for activating piezoresistive sensors in a matrix. The outputs of buffers, for which operational amplifiers can be used,are connected to the columns and rows.
European Patent Application No. 791 834 describes a method for determining the resistance of a resistor arrangement, in which leakage current is generated by applying voltage potentials of the same value. Furthermore, a measurement of the quantities required for determining the resistance value is performed by applying a first and a second voltage potential having different potential values to obtain a test current flowing through the resistor. These voltage potentials are selected so that the direction of the test current coincides with the direction of the cover current. Subsequently, the adjusted terminal voltages and a test current are measured. The resistance value is calculated using the measured values of the leakage current, test current, and terminal voltage.